Electron beam focusing



Filed April 29. 1953 *Oct.v7 1958 JQTQ MEVlxuDFl.Y v ELEcTRoN BEAMFQCUSING 2 Sheets-Sheet 1 ATTORNEY v.1. 1'. MENDEL ELEcTRoN BEAM FocUsING v oct. 7, 195s z sheets-sheet 2 lFiled April 29, 1953 VO vn v REG/0N d V316 FOR MMI/MUM R/PPLE O o A .1 o

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ELECTRON BEAM FOCUSIN G John T. Mendel, Summit, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 29, 1953, Serial No. 351,874

7 Claims. (Cl.` 315-3.6)

This invention relates to systems for kfocusing hollow beams of charged particles, and more particularly to .fp

systems in which a hollow electron beam is collimated by magnetic elds for travel over a relatively long path, such as is the case in some forms of traveling wave tubes.

A broad object of the invention is to provide an improved system for focusing hollow beams of charged parf circu'italn'g which the' wave is propagating. For eit,

cient operation, it is generally important to keep the electron ow cylindrical to avoid having the electronsstrike the interaction circuit and to conne the electrons to regions of high signal fields. Therefore, to minimize transverse components of ow resulting from the space charge effects in the beam, it is generally necessary to employ some form of focusing.

Hitherto it has generally been the practice in traveling wave tubes to focus the electron stream by means of a uniform longitudinal magnetic eld along the path of liow. However, both because of the relatively long length of the electron path and 4because of the large space charge forces acting in an electron stream of high density, it

has been found that the auxiliary equipment necessary' to provide a satisfactorily uniform magnetic eld is large and bulky, being many times the size and weight of the traveling wave tube alone. For obvious reasons, it is desirable to minimize the size and weight of this focusing equipment.

In J. R. PierceV Patent 2,847,607, issued August 12, 1958, there are described permanent magnet focusing arrangements, each of which employs a succession of regions of longitudinal axially symmetric magnetic field along the path of ow characterized in that the direction of the field reverses with successive regions. Good focusing has been achieved with a time-constant spatially alternating magnetic field of this kind at a considerable saving in size and weight of the focusing equipment.

The present invention relates primarily to modifications of the arrangements described by Pierce in the aboveidentified patent for better adapting them to the focusing of hollow electron beams. More particularly, the focusing arrangements ofthe present invention utilize` radial electrostatic fields which provide an outwardly directed force on the electrons to cooperate with the time-constant in traveling wave tubes as a result of the trend towardr higher frequencies. For example, in` helix-type tubes foryuse at millimeter `wavelengths where `the circumferinl ence of the helix preferably is approximately half the operating wavelength, it is found advantageous to employ a hollow electron beam which flows past the outside of the helix.

In accordance with the present invention, for focusing a hollow electron beam la tirne-constant spatiallyalternating magnetic ield is established along the path of electron liow, and additionally the flow is confined to the interspace between two cylindrical electrodes between which is set up a radial electrostatic field for providinga radial outward force, `one of the two electrodes preferably serving also as the interaction circuit for propagating the slow electromagnetic wave in coupling relation With the electron beam. By suitable adjustment of the strength of the magnetic and electrostatic elds, the hollow electron beam is focused.

The invention will be better understood from the following more detailed description taken in conjunction with the accompanying drawings in which:

Fig. l shows schematically an electron beam system in accordance with the invention;

Figs. 2 and 3 snow in schematic form traveling wave tubes incorporating hollow electron beam systems Yin accordance with the invention;

Fig. 4 shows a fragment of the path of electron ow and the associated electrodes of the system shown in Fig. 1 and will be helpful in an exposition of the principles of the invention; and

Fig. 5 is a plot of parameters important in the` design of a time-constant spatially alternating magnetic eld for use inthe practice of the invention.

Referring now particularly to the drawings, Fig. 1 illustrates schematically an electron beam system 10 in accordance with the invention suitable for incorporation in a traveling wave tube. At opposite ends of an evacuated envelope 11, which, for example, is of glass or a suitable non-magnetic metal such as copper, are positioned a source of a hollow electron beam 12 and a target 13; The source of electrons generally is an electron gun which includes an electron-emissive ring shaped cathode, a heater unit, an intensity control element, and an electrode system for shaping and accelerating the electron beam. The target 13 serves as the collector of the electrons at the end of their paths, and to this end ismaintained ata suitable positive potential with respect to the cathode of the electron source 12 by means of a voltage supply 17. TwoV cylindrical non-magnetic electrode members 15, 16 disposed coaxial. with the desired path of ow are positioned inside and outside, respectively, the hollow electron beam, i. e., the electron beam Hows through the interspace between inner and outer electrode members 15 and 16, respectively. In a traveling wave tube, it will generally be convenient to utilize at least one of the two cylindrical members as the slow wave circuit for propagating electromagnetic waves for interaction with the electron beam in the manner characteristic of traveling wave tube operation. The two electrodes 15 and 16 are each maintained by voltage supply 17 at a positive potential with respect to the cathode of the electron gun for providing accelerating electric fields. Additionally in accord-- ance with the invention, the outer electrode 16 is maintained at an appropriate positive potential with respect to the inner electrode 15 for providing an outwardly directed force on the electrons. Moreover, to provide a time-constant spatially alternating longitudinal magnetic eld, a regular succession of identical tubular cylinders 18 of magnetic material, such as soft iron or Alnico, are disposed coaxial with the path ofelectron flow along the elongated portion of the tube envelope. Successive cylinders are spaced apart leavinggaps 19 of uniform length between adjacent cylinders. A series of identical U- shaped permanent magnets 20 are disposed along the path of electron flow. For convenience of illustration adjacent magnets are shown disposed on diametrically opposite sides of the axis of the electron path. Successive magnets are bridged across successive gaps 19, each having separate pole faces kin good magnetic contact with a different one of two adjacent cylinders. Successive magnets are also reversed in sense, so that adjacent cylinders are in contact with oppositely-poled pole faces. Accordingly, the succession of cylinders may be Viewed as a succession of oppositely-poled pole pieces of a permanent magnet structure.

As a result of this succession of oppositely-poled pole pieces, there results along the path of electron ow a succession of regions 21, each corresponding to a gap 19 between cylinders, where there exists a longitudinal magnetic eld of substantiallyaxial symmetry, and the direction of this magnetic iield reverses along successive regions. By the cooperative effect of this spatially alternating magnetic eld and the radial electrostatic eld set up between the inner and outer electrodes, the hollow electron beam is focused.

Before describing in detail an illustrative embodiment of the invention, it will be helpful to analyze the principles of focusing a hollow electron stream by a timeconstant spatially alternating axially symmetric magnetic field and a radial electrostatic eld as is characteristic of the invention.

-In the above-identified Pierce patent, there are developed the principles of focusing a solid electron beam by a time-constant spatially alternating axially symmetric magnetic field. It is in accordance with one aspect of the invention to adjust the electric elds existing in the hollow beam by the use of applied radial electric fields to achieve the same spatial variation with radial distance r as is characteristic of a solid beam, so that thereafter the principles developed for focusing .a solid beam can be applied to the hollow beam. If it is assumed that the thickness of the hollow beam Ar is much smaller than r, which is the case of principal interest, then it can be shown that this desired condition on the electric elds in the hollow beam is realized when where V1 and V2 are the potentials of the inner and outer electrodes 15 and 16, respectively; ra and rb are the inner and outer radii of the hollow electron beam, respectively; r1 and r2 are the radii of the inner and outer electrodes, respectively; Q is the charge per unit length of` the electron beam; and e is the dielectric constant of the interspace region. In Fig. 4, there is shown a fragment of the electron path and its associated electrodes with the dimension labeled as above.

Equation l can be satisfied only approximately since the outside radius of the beam rb is a periodic function of distance down the tube, and there must be utilized therefor a mean value. However, even with these reservations, the potential dilference can be chosen so that the forces acting on the hollow beam are sufliciently identical to those acting in the case of the solid beam analyzed in the Pierce patent that the same analysis is applicable. Consequently, the problem of adjusting the strength and periodicity of the magnetic eld can be now solved by reference to the analysis for the periodically focused solid electron beam set vforth in the Pierce patent.

Such analysis indicates that if there is defined raz where a7 is the charge-mass ratio of an electron; e is the dielectric constant of the interspace; L is twice the magnet spacing or the length of one cycle of the magnetic field; B0 is the peak magnetic field along the path of flow; lo is the D.-C. beam current; un is the D.-C. beam velocity; and

p=1ruo(t"b2-Taz) (5) Then if there is defined further 1 wL 2 arri?) (6) where a is a magnetic eld intensity parameter, and is a space charge parameter, the relationship between a and for minimum ripple is plotted in Fig. 5. l

Accordingly, for focusing in accordance with the principles set forth a hollow electron beam of given design parameters, there can be derived by means of the curve shown in Fig. 5 the required design parameters of the time-constant spatially alternating magnetic eld structure and by means of Equation l the required deslgn parameters of the radial electric field structure.

However, due to the aberrations in the initial focusmg at the cathode and because of thermal velocities, some electrons will not be confined to theoretical laminar orbits and may drift towards the axis of the hollow beam. To suppress such stray electron flow, 1t may be advantageous in practice to increase somewhat the potential ditference between the inner and outer electrodes beyond that calculated in order to augment the outward force acting on such stray electrons.

It is also to be noted that it is possible that the stable solution analogous to the solid beam case with which the present analysis is concerned may not be the only stable solution. It does, however, represent a mathematically tractable solution.

Fig. 2 illustrates how a typical traveling wave tube can be adapted for use with a hollow electron beam system in accordance with the invention. In the interest of simplicity, the tube is shown in schematic form, many of the specific tube details being omitted. The various tube elements are enclosed in a non-magnetic envelope 30. Alternatively, it is possible to utilize an envelope of a magnetic material such as Kovar so long as it is made sufficiently thin to become readily saturated Without seriously reducing the magnetic eld. At one end to serve as a source of electrons there is positioned an electron gun 31 which comprises an annular electron-emissive cathode 32 and an electrode system 33 for shaping and focusing the electrons emitted into a convergent v conical annular beam. Suitable lead-in conductors are provided to the various elements of the electron gun from a suitable voltage supply source not shown. At the opposite end of the tube there is positioned a target electrode 34 for collecting the spent electrons.

The tube shown in Fig. 2 is adapted particularly for coaxial input and output connections to the slow wave interaction circuit which comprises a helically coiled conductor 35 coaxially disposed along the path of ow of the hollow beam and enclosed thereby. Input waves are applied by way of the coaxial input termination 36, of which the inner conductor 37 is an extension of the conductor 35. To this end, for example, the inner conductor 37 which is initially a solid rod is connected as an extension of a tubular portion which is grooved in a helical pattern, the pitch of the grooving increasing gradually for impedance matching purposes, for forming the helically coiled conductor 35. Moreover, the outer conductor 38 of the coaxial inputtermination is extended past the electron gun 31, having its diameter increased therepast, and an'extension thereof serves as the outer cylindrical eleotrode 39 surrounding the path of electron ow in accordance with the principles set forth in connection with the focusing system shown in Fig. 1. To this end, it is desirable to maintain the outer conductor 38 at a positive D.C. potential with respect to the inner conductor 37 by means of connections to voltage supply 40 whereby along the path of flow the helical conductor 35 is at a negative potential with respect to the outer electrode 39. However, both are maintained positive with respect to the electron source to provide a longitudinal accelerating eld on the electron fiow.

At the downstream end, the .amplified output wave is abstracted by way of output coupling coaxial connection 41 of which, in a manner analogous to the input coupling coaxial connection 36, the inner conductor 42 and the outer conductor 43 are extensions, respectively, of the helical conductors 35 and the cylindrical electrode 39.

The succession of regions lof longitudinal magnetic field of the kind desired for the practice of the invention can be achieved in any of the Ways described either in the aforementioned Pierce patent in Patent 2,841,739, 'issued July l, 1958, also of I. R. Pierce, or in P. P. Ciof Patent 2,844,754, issued `luly 22, 1958. By way of example, there is shown for use with this tube the magnetic structure shown in Fig. 1 which is one of the many possible alternatives. A succession of magnetic annular cylindrical members 44 surround the tube envelope spaced apart longitudinally. By means of permanent magnets 45, the successive cylinders are oppositely poled to serve as a succession of oppositely-poled pole pieces for creating along the path of flow a spatially alternating magnetic eld of the kind desired. The various parameters such as spacing and field intensities are adjusted in accordance with the principles set forth above.

Fig. 3 shows a traveling wave tube 50`which incorporates an embodiment of the invention in which the outer of the two coaxial cylindrical electrodes is a helical conductor 51 which serves as the slow wave interaction circuit while the inner is a rod 52 extending along the path of flow. A non-magnetic envelope 53 encloses the Various tube elements. At one end, an electron gun 54 serves as a source of a hollow electron beam, and at the yopposite end, a target electrode 55 collects the spent electrons. Dispo-sed coaxially along and surrounding the path of electron ow is the helically coiled conductor 51 which propagates the electromagnetic wave to be amplified. Additionally, disposed coaxially along and enclosed by the path of electron flow is the conductive rod 52. In accordance with the focusing principles set forth above, the conductive rod is maintained suitably negative with respect to the helical conductor by lead-in connections to the voltage supply 57.

This tube is adapted particularly for input and output hollow wave guide connections to the wave interaction circuit. To this end, the helix 51 is joined at opposite ends to input coupling strip 58 by an impedance matching section 59 and to an output coupling strip 60 by an impedance matching section 61. These matching sections are extensions of the helix in which the pitch is gradually increased. An input wave is applied to the upstream end of the interaction circuit by way of input. wave guide coupling connection 62 and the output wave is abstracted at the downstream end by Way of output wave guide coupling connection 63. Each of the wave guide coupling connections 62, 63 is a section of rectan` gular wave guide which has a pair of opposite side Walls` 62A, 62B and 63A, 63B apertured for passage therethrough of the tube envelope, and which has a closed end 62C, 63C and an open end 62D,' 63D by which it can be connected into a wave guide transmission system. Each of the input and output coupling strips 58, 60 is supported in its corresponding wave guide coupling connection. Input waves are applied to the input wave guide connection 62 to have a mode of propagation havconductive center' rod 52 are each maintained at a positive potential with respect to the electron source, by voltage supply 57, the center rod being more negative than the helix.

The desired succession of longitudinal magnetic field regions can again be achieved in any of the ways described in the aforementioned Pierce and Cioi patents. In this illustrative embodiment, a spaced succession of cylinders 64 of a magnetic material is disposed around the tube envelope to serve as a series of pole pieces. In the region of the input and output wave guide connections special steps are taken to minimize discontinuities in the periodically varying magnetic field desired. To this end, it is desirable that the two apertured side Walls 62A, 62B and 63A, 63B of each of the wave guides 62, 63 also be of a magnetic material while the other side walls and the end closures 62C, 63C of each be nonmagnetic. In this way, each of the apertured side walls can be made to serve as a separate pole piece along with the succession of cylinders 64. Between adjacent pairs of pole pieces there are bridged permanent magnets 65 in a way that successive pole pieces are oppositely poled, whereby there is provided along the path of flow a succession of regions of longitudinal magnetic field, the direction of the magnetic field reversing with successive regions. It should be evident that hollow beam focusing systems which in accordance with the invention are characterized by a magnetic structure for providing a periodically varying longitudinal magnetic field along the path of flow and an electrode system for providing a radial electrostatic field along the path of flow can be utilized in various other forms of traveling Wave tubes. Accordingly, it is to be understood that the specific embodiments described are merely illustrative of the principles of the invention. Various other arrangements can be devised by one skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. In a traveling wave tube, inner and outer cylindrical electrodes coaxially and concentrically disposed, at least one of said electrodes forming a slow wave circuit for a traveling electromagnetic wave, means for forming and projecting a hollow cylindrical electron beam coaxially with said cylindrical electrodes through the interspace therebetween, means for maintaining the outer electrode at a positive potential with respect to the inner electrode, and permanent magnet means disposed around said outer cylindrical electrode for forming a succession of regions of axial magnetic field along the path of flow,

the direction of said field reversing with successive regions.

2. In combination, first and second cylindrical members coaxially disposed, the first surrounding the second and defining an interspace therebetween, the first member being adapted to be at a positive potential with respect to said second member, means for projecting a hollow cylindrical electron beam coaxial with said cylindrical members through said interspace, and permanent magnet means disposed around said cylindrical members for forming along the path of said cylindrical beam a succession of regions of axial magnetic field, the direction of the magnetic field reversing with successive regions.

3. In combination, rst and second cylindrical members coaxially and concentrically disposed, the outer of said cylindrical members being adapted to be at a positive potential with respect to the inner of said cylindrical members, means for projecting a hollow cylindrical electron beam coaxial with cylindrical members through 7 4theinterspace formed therebetween, and magnetic means disposed aroundsaid cylindrical .members for producinga Atime-constant spatially :alternating axial magnetic -eld ,along the 4path lof yflow of the electron beam.

4. ln a traveling wave tube, means forming a hollow cylindrical ,electron ibeam, .a Aconductor helically -coiled disposed coaxial with and surrounding the path of ow of said beam, af/conductive member disposed coaxial with and surrounded by the path of flow of said beam, the helical conductor adapted to be positive with respect to `said member, and permanent magnet means disposed around said helically coiled conductor for forming along :thepath ofllow atime-constant spatially alternatingy axial 6. In a traveling wave tube, means forming a hollow cylindrical-electron beam, inner and outer cylindrical electrodes vkcoaxially disposed along the path of flow of said beam, one of said electrodes being an interaction wave circuit for propagating an electromagnetic wave, means for maintaining the inner electrode at a negative potential with respect to the outer electrode, a plurality of cylindrical magnetic elements spaced apart along and surrounding the Ypath y,of ow, `and permanent magnet Ameans in magnetic `contact with said elements oppositely poling successive elements.

7. In a traveling wave tube, an annular electron source and a target defining a path of ow for a hollow cylindrical electron bee-m, inner `and ,outer cylindrical electrodes coaxially .disposed :along the path of ow and defining therebetween ,au interspace region for the path of flow, a voltage source Vhaving positive and negative terminals, the positive terminal being connected -to the outer electrode, the negative terminal being connected to the inner electrode, a succession of magnetic cylinders spaced apart along and surrounding the path of flow, andlpermanent magnet means oppositely poling successive cylinders.

References Cited in the tile of this patent UNlTED STATES PATENTS 2,200,039 Nicoll May 7, 1940 2,300,052 Lindenblad Oct. 27, 1942 2,305,884 Litton Dec. 22, 1942 2,503,173 Reisner Apr. 4, 1950 2,707,759 Pierce May 3, 1955 2,742,588 Hollenberg Apr. 17, 1956 OTHER REFERENCES YCourant et al.: The Strong-Focusing Synchroton-A New High Energy Accelerator, Physical Review, vol. 88, No. 5, December `1952, pages 1190 to 1196. 

